discrimination between electron and nuclear recoils in ......recoils in dark matter direct detection...
TRANSCRIPT
Discrimination between
Electron and Nuclear Recoils
in Dark Matter DetectorsBy: Vetri Velan
September 21, 2016
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Dark Matter Direct Detection
• Basic principle of a DM search is to observe a dark matter particle (in this talk, WIMPs) interacting with a Standard Model
particle
• Direct detection experiments search for recoils of a galactic
WIMP with an atom
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𝜒
𝜒
Dark Matter Direct Detection
ER: e-, µ-, γ
NR: n, WIMP
These two processes often produce similar signals, so it is necessary
to “discriminate” between the two to reduce backgrounds
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Energy deposits in material
3 primary channels through which energetic particles deposit their energy
in matter:
Ionization (charge)
Scintillation (light)
Heat (phonons)
Direct detection experiments attempt to detect 1 or 2 of these channels
By detecting 2 channels, we are able to discriminate between nuclear
recoils (NR) and electronic recoils (ER)
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Energy deposits in material5
Source: Ref. [1]
Two-phase liquid noble element
time projection chambers6
Heat
LightCharge
Source: Ref. [2]
Capable of measuring both scintillation light
and ionization electrons
Detectors consist of:
A chamber of noble liquid (usually Xenon or
Argon), with a gas phase region above the
liquid
Photon detectors (typically photomultiplier
tubes) surrounding the liquid region
An electric field (“drift field”) in the liquid, and
a stronger “extraction field” in the gas
At left: general schematic of interactions in
LUX
Two-phase liquid noble element
time projection chambers7
Source: Ref. [2]
Primary scintillation light (S1) produced at the
interaction site, detected by PMTs at the top
and bottom of detector
Ionization electrons drift up through the liquid
xenon, in the drift field
Some recombine with positive ions, releasing
more scintillation light (S1)
Others are extracted above the liquid surface,
into gas phase region, where they form
secondary proportional light (S2)
Time between S1 and S2 gives us z-position of
the recoil
Pattern of S2 light on the PMTs gives us xy
Heat
LightCharge
Two-phase liquid noble element
time projection chambers8
Source: Ref. [2]
Discrimination: the ratio of S2/S1 is different
for electronic recoils and nuclear recoils
Nuclear recoils have denser tracks, so they
have more electron-ion recombination, and
thus a lower S2/S1
Crucially, this quantity is independent of
particle ID—it depends on recoil type,
energy, and detector properties
Heat
LightCharge
Two-phase liquid noble element
time projection chambers9
How do we actually
discriminate (i.e. given
a recoil, tell whether it is
NR or ER)?
Answer: Calibration!
Use known sources of
β and γ radiation to
calibrate ER, and
sources of neutrons to
calibrate NR
At left: Calibration
results from a Columbia
detector (AmBe for n,
Cs-137 for γ)
Heat
LightCharge
Source: Ref. [3]
Two-phase liquid noble element
time projection chambers10
How is this used in an analysis?
Lux 2013:
ER calibrated with tritiated
methane CH3T, a β source
NR calibrated with AmBe and
Cf-252, neutron sources
Discrimination power of 99.6%
Heat
LightCharge
Source: Ref. [4]
Two-phase liquid noble element
time projection chambers11
Lux 2013:
WIMP search signal region, with 118 kg of fiducial mass and 85.3 live-day exposure
Backgrounds include external γ, radio-isotopes in the detector, and neutrons
Another background is leakage from ER events into the NR band—in this case, 0.64 ± 0.16 events
Use these background expectations and results in a profile-likelihood-ratio to set limits on DM interactions
Heat
LightCharge
Source: Ref. [4]
Two-phase liquid noble element
time projection chambers12
Heat
LightCharge
The themes that were presented for discrimination in dual-phase
TPCs are going to be valid in other detection techniques as well
Identify the channels of energy deposit; analyze the apportionment
of energy into the different channels
Use calibration to separate NR signals from ER signals
Use this discrimination to reject ER backgrounds, which are usually
much more common than NR backgrounds
Cryogenic bolometers with
charge readout13
Heat
LightCharge
To see how heat and charge
channels in cryogenic
bolometers can be used
simultaneously to discriminate,
we’ll use CDMS-II as a case study
The detector in CDMS-II is called
a Z-Sensitive Ionization and
Phonon (ZIP) detector (see left)
Cryogenic crystal made of silicon
or germanium
Source: Wikipedia
Cryogenic bolometers with
charge readout14
Heat
LightCharge
Ionization signal:
Some portion of the recoil energy creates e-/h+ pairs in the crystal, which form a
cascade of e-/h+ in the conduction band
Drift in an electric field towards electrodes
Phonon signal:
Prompt phonons, generated from instantaneous displacement of nuclei and
electrons
Recombination phonons from charge carriers reaching the electrodes (see above)
Luke phonons: energy dissipated in the crystal from the electric field doing work
Phonons measured by transition-edge sensors (TES), >4000 in each ZIP, connected
to SQUIDs
Cryogenic bolometers with
charge readout15
Heat
LightCharge
We expect ER to deposit more
of their energy as ionization,
compared to NR; this is exactly
what we see
Discriminating variable is
ionization yield = EQ/ER
EQ is the “electron-equivalent”
ionization energy
ER is the recoil energy
ER calibration from 133Ba (bands
are ±3σ), NR calibration from 252Cf (bands are ±2σ)
Source: Ref. [5]
Cryogenic bolometers with
charge readout16
Heat
LightCharge
Backgrounds are:
Electron recoils in the
bulk of the material,
caused by radiogenic
isotopes in the
detector (see left),
discriminated by
ionization
Neutrons from internal
sources or from
cosmic ray-induced
spallation, reduced by
going underground
and muon veto shield
(See next slide)
Source: Ref. [5]
Cryogenic bolometers with
charge readout17
Heat
LightCharge
Backgrounds are:
ER at the edge of the
detectors,
discriminated by
timing properties of
the phonon signal
(see left)
Source: Ref. [5]
Scintillating cryogenic bolometers18
Heat
LightCharge
Cryogenic Rare Event Search with
Superconducting Thermometers
(CRESST) is an example of a DM
search that uses phonons and
photons as signal channels
As in other cryogenic bolometers,
phonons propagate through crystal
and are detected by TES
CRESST uses scintillating CaWO4
crystals, in conjunction with a
silicon/sapphire wafer and TES, to
measure photon signal
Scintillating cryogenic bolometers19
Heat
LightCharge
Light yield: ratio of light to phonon
signal
57Co for ER calibration (122 keV γ)
NR calibration with neutron source
Able to use quenching factors
measured elsewhere, to determine
NR bands for recoils of oxygen,
tungsten, and calcium
We’ve finished all possible combinations of
energy channels, so we’re done, right…?
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Heat
LightCharge
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Pulse-Shape Discrimination
Liquid noble elements scintillate by
forming excimers E2+ , which then de-
excite with a characteristic timescale
Singlet and triplet states have different
time constants
Triplet decays are suppressed in
nuclear recoils, due to Penning
ionization and spin exchange
So this is a valid approach for
discrimination, using only one channel
of energy deposit
Xe: 𝜏1 = 4 𝑛𝑠, 𝜏3 = 22 𝑛𝑠
Ar: 𝜏1 = 7 𝑛𝑠, 𝜏3 = 1600 𝑛𝑠
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Source: Ref. [8]
Methods of Pulse-Shape Discrimination
1. Prompt Fraction Method
Define 𝑓𝑝 =𝑇𝑖
𝜉𝑉 𝑡 𝑑𝑡
𝑇𝑖
𝑇𝑓𝑉 𝑡 𝑑𝑡
Use this as discrimination
variable
At left, results from a single-
phase LAr detector (3.14 L
active volume).
Here, 𝜉 = 90 𝑛𝑠 𝑇𝑖 = 𝑡0 − 50𝑛𝑠, 𝑇𝑓 = 𝑡0 + 9000 𝑛𝑠, and 𝑡0 is the
trigger time (empirically
determined to give the best
results).
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Methods of Pulse-Shape Discrimination
2. Multibin method
Bin signal time and fraction of
detected photoelectrons into
K x L bins
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Methods of Pulse-Shape Discrimination
For given experiment, multibin
method is better—there might
be other algorithms
Dark matter Experiment with
liquid Argon and Pulse shape
discrimination (DEAP-3600)
aiming to use PSD in LAr, based
on previous success in DEAP-1
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Conclusions
To reduce backgrounds (primarily from electrons and gamma rays), it is
important to be able to discriminate between electron recoils and nuclear
recoils in dark matter direct detection
Noble liquid TPC’s and cryogenic bolometers have been successful at this
by looking at the ratios between two energy channels
Other forms of discrimination exist that only use one channel of energy
deposit, such as pulse-shape discrimination and annual modulation
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Questions?
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Sources1. T. M. Undagoitia and L. Rauch. arXiv:1509.08767v1. 26 Sep 2015.
https://arxiv.org/pdf/1509.08767v1.pdf
2. https://www.hep.ucl.ac.uk/darkMatter/
3. http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.97.081302
4. D.S. Akerib et. al. (LUX Collab.) arXiv:1310.8124v2. 5 Feb 2014. http://arxiv.org/pdf/1310.8214v2.pdf
5. S. Fallows. Univ. of Minnesota Thesis, Dec 2014. http://cdms.berkeley.edu/Dissertations/fallows.pdf (S. Fallows thesis)
6. G. Angloher et. al. (CRESST Collab.) arXiv:1509.01515v2. https://arxiv.org/pdf/1509.01515v2.pdf
7. http://link.springer.com/article/10.1140/epjc/s10052-012-1971-8
8. W. H. Lippincott et. al. arXiv:0801.1531v4. 23 Sep 2008. https://arxiv.org/pdf/0801.1531v4.pdf
9. P. Phelps. CWRU Thesis, Aug 2014. https://etd.ohiolink.edu/!etd.send_file?accession=case1404908222&disposition=inline
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